1. Field of the Invention:
This invention relates to a semiconductor integrated circuit having at least two circuit blocks therein which are independently energized through different power supply terminals and, more particularly, to a protection structure incorporated therein against an application of abnormally high electrostatic voltage to signal terminals.
2. Description of Related Arts:
In a semiconductor integrated circuit having plural number of circuit blocks each including an electric circuit of MOS field effect transistors, a power is supplied to energize respective circuit blocks through different power supply terminals which are allocated to the respective circuit blocks and formed on the same IC chip. This structure is advantageous in suppressing noise on power wiring that is generated by a current flowing through the power supply wiring and an impedance thereof. The separated power supply terminals decrease the current flowing through each power supply wiring to reduce the voltage change on the power supply wiring based on a change in the current. Such voltage change is applied to the circuit block as the noise.
In the separated power supplying structure, either high or low power voltage terminals are respectively connected to different power wirings, and the remaining power voltage terminals are connected through semiconductor substrate. Moreover, the chip is provided with signal terminals such as input terminals and output terminals which are mainly protected from applicable electrostatic charges by gate protection diodes connected between the respective signal terminals and the power terminals.
If abnormally large electrostatic charges are accumulated on a signal terminal and cause a flow of current between power supply or signal terminals of different circuit blocks, that current transmits through the semiconductor substrate. The resistance of the semiconductor substrate in the discharging path is from several hundreds ohms to several tens hilo-ohms to prolong the time of discharge. The resistance of the substrate generates a large voltage drop, and produces a large difference in electric potential on the substrate. Therefore, a large voltage is applied to gate insulator films of MOS field effect transistors connected to the signal terminal and breaks them down.